The rotating anode of an X-ray tube typically is mounted upon bearings as a means of rotation about an axis. In order to ensure a consistent rotatability over the life of the tube, lubrication is added in some form to the bearings. Use of a liquid is not feasible since it easily evaporates within a short time in the evacuated tube once its boiling point has been reached. Therefore, a solid lubricant is preferred. Soft metal lubricants such as silver, gold and some rare earth metals such as an ultra thin (1 micron or less) layer of titanium carbide have been proposed.
In order to remove gases incorporated into the anode and other tube components during manufacture, it is necessary to subject the tube to a high temperature bakeout under vacuum. This is especially necessary where the cathode is porous and has been impregnated with an electron emissive material; an "impregnated cathode" type tube. The standard procedure has been to bakeout the entire vacuum tube at 425.degree. or less. Better liberation of gaseous impurities, however, occurs under higher temperatures. In general the more complete the bakeout, the longer will be the useful life of the tube.
One limitation to subjecting the tube to higher temperatures has been the glass casing of the vacuum tube itself. Now casings are composed of Pyrex with Kovar fittings. A further limitation, however, existed in the fact that at higher bakeout temperatures, even though they are below the melting point of the lubricant metal, the bearing sometimes freezes up under the weight of the anode itself when subjected to these higher temperatures. The combined action of a temperature close to the melting point of the metal lubricant and the pressure caused by the stationary anode produces a cold braze of the bearings.